Bilaterally coherent tremor resembling enhanced physiological tremor: report of three cases

Physiological Recordings of the Cerebellum in Movement Disorders

The cerebellum plays an important role in movement disorders, specifically in symptoms of ataxia, tremor, and dystonia. Understanding the physiological signals of the cerebellum contributes to insights into the pathophysiology of these movement disorders and holds promise in advancing therapeutic development. Non-invasive techniques such as electroencephalogram and magnetoencephalogram can record neural signals with high temporal resolution at the millisecond level, which is uniquely suitable to interrogate cerebellar physiology. These techniques have recently been implemented to study cerebellar physiology in healthy subjects as well as individuals with movement disorders. In the present review, we focus on the current understanding of cerebellar physiology using these techniques to study movement disorders.

Slow Orthostatic Tremor: Review of the Current Evidence

Background

Orthostatic tremor (OT) is defined as tremor in the legs and trunk evoked during standing. While the classical description is tremor of ≥13 Hz, slower frequencies are recognized. There is disagreement as to whether the latter represents a slow variant of classical OT, or different tremor disorder(s) given frequent coexistent neurological disease.

Methods

A systematic literature search of PubMed was performed in February 2019 for “slow orthostatic tremor” and related terms which generated 573 abstracts, of which 61 were included.

Results

Between 1970 and 2019, there were 70 cases of electrophysiologically confirmed slow OT. Two-thirds were female, of mean age 60 years (range 26–86), and mean disease duration 6 years (range 0–32). One-third of cases were isolated, and two-thirds had a coexistent disorder(s), including parkinsonism (30%), ataxia (12%), and dystonia (10%). Postural arm tremor was present in 34%. Median tremor frequency was 6–7 Hz (range 3–12). Tremor bursts ranged from 50 to 150 ms duration, and were alternating or synchronous in antagonistic and/or analogous muscles. Low and high coherences were reported. Five cases (7%) had coexistent classical OT. Clonazepam was the most effective medication across all frequencies, and levodopa was effective for 4–7 Hz OT with coexistent parkinsonism. Two cases resolved with the treatment of Graves’ disease. Electrophysiology and imaging predominantly support a central tremor generator.

Discussion

While multiple lines of evidence separate slow OT from classical OT, clinical and electrophysiological overlap may occur. Primary and secondary causes are identified, similar to classical OT. Further exploration to clarify these slow OT subtypes, clinically and neurophysiologically, is proposed.

Spinal interneuron circuits reduce approximately 10-Hz movement discontinuities by phase cancellation

Tremor imposes an important limit to the accuracy of fine movements in healthy individuals and can be a disabling feature of neurological disease. Voluntary slow finger movements are not smooth but are characterized by large discontinuities (i.e., steps) in the tremor frequency range (approximately 10 Hz). Previous studies have shown that these discontinuities are coherent with activity in the primary motor cortex (M1), but that other brain areas are probably also involved. We investigated the contribution of three important subcortical areas in two macaque monkeys trained to perform slow finger movements. Local field potential and single-unit activity were recorded from the deep cerebellar nuclei (DCN), medial pontomedullary reticular formation, and the intermediate zone of the spinal cord (SC). Coherence between LFP and acceleration was significant at 6 to 13 Hz for all areas, confirming the highly distributed nature of the central network responsible for this activity. The coherence phase at 6 to 13 Hz for DCN and pontomedullary reticular formation was similar to our previous results in M1. By contrast, for SC the phase differed from M1 by approximately pi rad. Examination of single-unit discharge confirmed that this was a genuine difference in neural spiking and could not be explained by different properties of the local field potential. Convergence of antiphase oscillations from the SC with cortical and subcortical descending inputs will lead to cancellation of approximately 10 Hz oscillations at the motoneuronal level. This could appreciably limit drive to muscle at this frequency, thereby reducing tremor and improving movement precision.

Fatigue-related changes in motor-unit synchronization of quadriceps muscles within and across legs

Two experiments were conducted to examine effects of muscle fatigue on motor-unit synchronization of quadriceps muscles (rectus femoris, vastus medialis, vastus lateralis) within and between legs. We expected muscle fatigue to result in an increased common drive to different motor units of synergists within a leg and, hence, to increased synchronization, i.e., an increased coherence between corresponding surface EMGs. We further expected fatigue-related motor overflow to cause motor-unit synchronization of homologous muscles of both legs, although to a lesser extent than for synergists within a leg. In the first experiment, different levels of fatigue were induced by varying posture (knee angle), whereas in the second experiment fatigue was induced in a fixed posture by instructing participants to produce different force levels. EMG coherence was found in two distinct frequency bands (6-11 and 13-18 Hz) and was higher within a leg than between legs. The fatigue-related increase of 6-11 Hz inter-limb synchronization resembled the increased motor overflow during unimanual contractions and thus hinted at an increase in bilateral coupling. Synchronization at 13-18 Hz was clearly different and appeared to be related to posture.

Clinical and neurophysiological characterization of myoclonus in complex regional pain syndrome

The origin of myoclonus in patients with complex regional pain syndrome (CRPS) is unknown. Eight patients with CRPS related myoclonus were clinically evaluated and studied with intermuscular and corticomuscular coherence analysis. Jerks were present at rest, aggravated during action and were frequently associated with tremulousness or dystonia. Electromyography demonstrated a burst duration ranging from 25 to 240 ms with burst frequencies varying from <1 jerk/s during rest to 20 Hz during action. Coherence studies showed increased intermuscular coherence in 4 patients in the 6 to 12 Hz band, as reported in patients with enhanced physiological tremor. In 2 patients side-to-side coherence was observed, pointing to a central oscillatory drive. Significant coherence entrainment was detected in 5 patients. We conclude that the characteristics of myoclonus in CRPS are different from other forms of myoclonus.

Bilateral tremor relations in Parkinson's disease: effects of mechanical coupling and medication

The aim of the present study was to examine the dynamics of bilateral multiple segment resting and postural tremor in 12 young, older subjects and eight Parkinson's (PD) patients in their different medication states. A second aim was to investigate whether bilateral independence of upper limb tremor was preserved for PD patients with amplified tremor under conditions where no upper limb segment was supported. Under these conditions, the likelihood for mechanical transmission between segments was increased. Tremor was recorded, bilaterally, from the hand and finger segments of all subjects. In addition to the PD subjects exhibiting greater tremor under both on/off medication states than the young/old control subjects, the tremor increase within limb from the hand to the finger was 2-3 times greater for the PD group in comparison to the healthy subjects. Despite this increased tremor, no differences were observed in the level of coupling between limbs across groups. Furthermore, the degree of coupling between limbs for the PD group was unaffected by their medication state. Overall, these results demonstrate that bilateral independence of tremor in PD participants is preserved despite conditions which maximised the chance of increased coupling.

Time-frequency analysis of transient neuromuscular events: dynamic changes in activity of the subthalamic nucleus and forearm muscles related to the intermittent resting tremor

In order to investigate the dynamic change in transient neuromuscular events and the functional correlation between the neural and muscular activity, local field potentials (LFPs) of the subthalamic nucleus (STN) and surface electromyograms (sEMGs) over several episodes of transient resting tremor from a patient with Parkinson's disease were quantitatively characterised in time-frequency domain using short-time Fourier transform and continuous wavelet transform. Events of onset and cessation of the tremor-related activity in the STN and muscles were correlated to reveal the temporal relationship between the two signals. A significant suppression in the power of the STN LFPs in the beta band (10-30 Hz) preceded the onset of resting tremor, which was presented as the increases in the power at the tremor frequency (3.0-4.5 Hz) in both STN LFPs and surface EMGs. Over the episodes of the intermittent resting tremor, the power of the STN LFPs in the beta band and the power of sEMGs in the tremor frequency band change in an alternating pattern with a significant exponential correlation (P(STN) = 16.8+62.3 x exp(-P(EMG)/6270.7); R2 = 0.72; p < 0.05). Significant linear correlation in the power values at the tremor frequency appears between STN LFPs and sEMGs (P(STN) = 65.1 + 2.1 x 10(-4)P(EMG); R2 = 0.41; p < 0.05). In comparison with short-time Fourier transform, similar results could be achieved using continuous wavelet transform of an appropriate wavelet with a higher temporal resolution but larger distortion in the high frequency.

Acoustic startle evokes bilaterally synchronous oscillatory EMG activity in the healthy human

Despite animal evidence that the reticulospinal system is of major importance to movement, this motor pathway has remained relatively inaccessible to experimentation in the human. Consequently, little is known about its function in health and disease. Here, we use the acoustic startle response to demonstrate that one type of reticulospinal activity in the human is associated with a characteristic pattern of bilateral synchronization between motor units. Surface electromyography (EMG) was recorded from upper limb muscles in 15 healthy subjects during the reflex startle to unexpected acoustic stimulation, voluntary movements mimicking the startle and during sustained voluntary tonic contraction. Frequency analysis demonstrated autospectral peaks at approximately 14 Hz in deltoid and biceps muscles only during the startle reflex. Similarly, coherence spectra of the EMG recorded between homologous proximal upper limb muscles demonstrated a peak centered approximately 12-16 Hz during reflex startles. Coherence in the 10- to 20-Hz band was significantly greater in the startle reflex than during voluntary sham startles or voluntary tonic contraction for deltoid, but not first dorsal interosseous, muscles. The coherence at 10-20 Hz between EMGs from homologous muscles represents a potential surrogate measure of reticulospinal activity that may be useful in determining the contribution of the reticulospinal system to different types of movement in health and disease.